Update build logic to a real build system

[JasonVMo]

This changes the build logic to use just + beachball. The initial source was copied from fabric but the change has some additional twists:

• Changed jest tests to use babel vs. ts-jest. The only thing lost here is type checking which is already covered by the typescript build.
• Added webpack.config.js files as appropriate to get things building. Webpack config is using the just framework rather than the bespoke approach of fabric
• Moved demo from packages to apps. This follows the approach of fabric.
• Added webpack.serve.config.js to get that running correctly

This also includes the incremental build support Ken added for Fabric with his lerna change. Now builds will clean, lint, build, test, and run webpack.

It is likely there are still places where the build logic can be simplified to be closer to standard approaches. Wherever we can cut things out we should unless there is a good reason for them.

Issues fixed in this change:

• Issue Add just support #3 Add just support
• Issue Add incremental build support #5 Add incremental build support

I also opened Issue #16 to cover switching from tslint to eslint

[kenotron]

Some comments left about the extraneous change file conversion scripts. In general, looks okay... since they're from UI Fabric web :)

[kenotron]

BTW, are these changes really needed? We shouldn't need to specify index like this. If we find that the compilation fails because of this, we should fix the configs.

[JasonVMo]

At some point I was hitting errors saying that these weren't targeting a physical file. I think in Fabric there is a forwarding file for controls. So there is a Pressable.ts that imports Pressable/index (I think).